MAP kinase pathways are conserved signal transduction pathways that activate transcription factors, translation factors and other target molecules in response to a variety of extracellular signals. Each pathway contains a MAP kinase module, consisting of a MAP kinase or ERK, a MAP/ERK kinase (MEK), and a MEK kinase (MEKK). In higher eucaryotes, activation of MAP kinase pathways has been correlated with cellular events such as proliferation, oncogenesis, development and differentiation. Accordingly, the ability to regulate signal transduction via these pathways could lead to the development of treatments and preventive therapies for human diseases associated with MAP kinase pathways, such as inflammatory diseases, autoimmune diseases and cancer.
Several MAP kinase pathways have been found in S. cerevisiae (Hunter and Plowman, Trends in Biochem. Sci. 22:18-22, 1997), and parallel mammalian pathways have been identified based upon sequences of mammalian ERKs and yeast MAP kinases, KSS1 and FUS3 (Boulton et al., Science 249:64-67, 1990; Courchesne et al., Cell 58: 1107-1119, 1989; Elion et al., Cell 60:649-664, 1990). The best delineated yeast MAP kinase pathway, activated by mating pheromones, is controlled by a receptor-G protein system, includes a Cdc42 small G protein, and requires at least three protein kinases, Ste20p (Leberer et al., EMBO J. 11:4815-4828, 1992; Ramer et al., Proc. Natl. Acad Sci. USA 90:452-456, 1993), Ste11p (Rhodes et al., Genes Dev. 4:1862-1874, 1990), and Ste7p (Teague et al., Proc. Natl. Acad. Sci. USA 83:7371-7375, 1986), upstream of the MAP kinase Fus3p (Elion et al., Cell 60:649-664, 1990).
Ste20p was isolated from S. cerevisiae as a gene whose product functions downstream of the .beta.y subunits of a heterotrimeric G protein but upstream of enzymes in the MAP kinase module (MEKK, MEK, ERK) of the pheromone response pathway (Leberer et al., EMBO J. 11:4815-4828, 1992; Ramer et al., Proc. Natl. Acad. Sci. USA 90:452-456, 1993). Ste11p, the MEKK, may be one of the Ste20p substrates (Wu et al., J. Biol. Chem. 270:15984-15992, 1990); thus, Ste20p-like enzymes may activate MEKKs in mammalian MAP kinase pathways. Ste20p, like its best studied mammalian counterparts, the p21-activated protein kinases (PAKs), is thought to be regulated by binding to Cdc42 through a conserved Cdc42/Rac interactive binding region, or CRIB domain (Burbelo et al., J. Biol. Chem. 270:29071-29074, 1995).
Mammalian relatives of Ste20p are diverse and include the PAK subfamily (PAK1,2,3) and the mixed lineage kinase (MLK) subfamily, including the dual leucine zipper kinase (DLK), germinal center kinase (GCK), and the Nck-interacting kinase, NIK. In the past year, newly identified Ste20p-related kinases include members of the MLK subfamily, SOK-1, Krs-1 and -2, and MUK. MUK was isolated in a screen for MEKK isoforms, but in fact shows more identity to MLK. In transfected cells several of these enzymes, as first shown with GCK, increase the activity of the stress-responsive kinases, particularly SAPK/JNK. In the case of NIK and GCK, they may work by binding to MEKK (Su et al., EMBO J. 16:1279-1290, 1997). However, several of these Ste20p-related enzymes also have MEKK activity. For example, DLK phosphorylates and potently activates MEKs that lie in the stress-responsive cascades.
Further characterization of members of these pathways, and the identification of additional members, is critical for understanding the signal transduction pathways involved and for developing methods for activating or inactivating MEKs and MAP kinase pathways in vivo. Accordingly, there is a need in the art for improved methods for modulating the activity of members of MAP kinase pathways, and for treating diseases associated with such pathways. The present invention fulfills these needs and further provides other related advantages.